1. Field of the Invention
This invention relates to microwave filters and, more particularly, to coupling mechanisms between transmission lines and resonators to provide improved power handling capability for microstrip/stripline type bandpass filters that are realized using high temperature superconductive materials. Further, this invention relates to a new coupling mechanism between input/output lines and resonators and between two adjacent resonators.
When resonators and transmission lines are referred to in this application, they can be either microstrip or stripline resonators and transmission lines.
2. Description of the Prior Art
Typical microstrip bandpass filters consist of input/output couplings, or I/O couplings and resonators where an I/O coupling consists of a feed line and an interface structure that provides a path from the feed line to the filter resonators. I/O couplings are also referred to as input/output terminations. An I/O coupling may be in the form of direct contact or gap coupled. FIGS. 1 to 3 show examples of microstrip bandpass filters with different I/O coupling types (see K. Chang, "Handbook of Microwave and Optical Components, Vol 1: Microwave Passive and Antenna Components", John Wiley & Sons, 1989). Conventional gap I/O coupling is either parallel-coupled or end-coupled, as shown in FIG. 2 and FIG. 3, respectively. Parallel coupled structure realizes coupling at one side of the resonator. It is suitable for long and narrow shaped resonator structures. To overcome the limitation of the feed line width which is determined by feed line impedance, a T-shaped end-coupling structure can be used, as shown in FIG. 4.
In high power applications using HTS thin film technology, wider resonators can be used to lower current density. The current density can be further reduced using sliced resonators (see co-pending U.S. patent application Ser. No. 08/595,864, now U.S. Pat. No. 5,922,650, issued Jul. 13, 1999), as shown in FIG. 5. However, to obtain desired I/O coupling, the end coupling structure described in the co-pending application requires a very small gap, which can cause arcing. Further, T-shaped end coupling structures (as shown in FIG. 4) can contain bend discontinuities where high current concentration exists.